I am trying to put insulation above a garage but the problem is the space in between the cross beams are different. They varying between 16", 17", 18". What would be the best way to insulate this space using batt without using blow-in insulation?

I have only found widths of 16" and 15" using R30 for attics.

enter image description here

  • You could cut it strips of 16", 17"or 18" to make it fit, but that would be a heck of job. Probably the way with the least amount of material wasted though. So if time is no constraint this might be your option.
    – Luuklag
    Dec 21, 2021 at 22:26
  • Blown in cellulose is the right answer, even though it's not what you want to do. :-)
    – Ecnerwal
    Dec 22, 2021 at 2:25
  • can't do blown in because I have a mini-split. The cellulose will get into the unit.
    – amrog
    Dec 22, 2021 at 17:40
  • @amrog it looks like you had blown in previously. One simple solution would be to lay down an air barrier before blowing in the insulation. That should prevent insulation from being sucked into the mini split unit. Dec 22, 2021 at 18:00
  • @RibaldEddie, what do you mean by an air barrier?
    – amrog
    Dec 22, 2021 at 22:51

4 Answers 4


Another option would be to cut the bats into 16-18" lengths (depending on the width of the specific joist cavity you're trying to fill) and run them crosswise in the joist cavities. You'll then end up with a quite a few pieces to fit in, but they'll all fit your different widths as you cut them to length to fit.


You should be able to find 23" batts for use in 24" OC framing, and then you would cut them down to fit.

But that would result in a lot of cutting, I would rather just blow insulation in.

  • does it matter faced or unfaced in an open space above the garage?
    – amrog
    Dec 22, 2021 at 17:41
  • 1
    I'm not an expert, but I would go with unfaced.
    – Glen Yates
    Dec 22, 2021 at 17:48

Two solutions;

Use polyester batts which can be ripped quickly leaving a straight edge.


Place batts at right angles over the joists, capturing an extra air pocket underneath. Much better insulation. Local code may require some indicator pegs (drywall) sticking up to mark where the joists are.


We cut bats to fit.

Either a thin sliver to fill the edge, or cut the bats such one half fits and the other needs a sliver.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.